1. Field of the Invention
The present invention relates to the field of powder metallurgy. More particularly, the present invention relates to processes for sintering aluminum powder. The present invention also relates to sintered aluminum articles.
2. Description of the Related Art
Aluminum's light weight, good strength, and corrosion resistance make it a desirable material for many applications. Techniques now exist for making aluminum powder in sizes useful in powder metallurgical forming processes. Such aluminum powder may be essentially pure aluminum or an aluminum alloy. Powder metallurgical forming processes permit an article to be made to its final shape or near to its final shape with little or no machining, thus reducing overall processing costs and material waste.
Aluminum reacts very quickly and strongly with oxygen to form alumina (Al2O3), so quickly that an alumina film immediately forms whenever an aluminum surface is exposed to even small amounts of atmospheric oxygen or water vapor. The alumina film is very thin, its thickness typically being in the range of about 30 to about 90 nm. The alumina film is nearly impervious to the migration of both aluminum and oxygen. Once an alumina film is formed, it is very stable and resistant to chemical attack, because few elements react more readily with oxygen than aluminum does.
Thus, when aluminum powder is made, a thin, protective alumina film forms all over the surface of each aluminum powder particle. The alumina film presents a problem in making powder metallurgical parts from aluminum powder. In order to join individual aluminum powder particles together to form a useful article, it is necessary for the metal atoms in one powder particle to come into direct contact with the metal atoms of the adjacent particles to form metallurgical bonds between them and to permit the atoms to migrate between the particles and to rearrange themselves along the interparticle boundary. The alumina films on the powder surfaces, however, interfere with the interparticle atom migration.
Fortunately, persons working in the art of powder metallurgy have found several ways of overcoming the problem presented by the alumina film on aluminum powder particles. One way is to mechanically rupture the alumina films and press the underlying metal of adjacent powder particles together by plastically deforming the aluminum powder particles through the application of mechanical force. For example, aluminum powder can be metallurgically bonded together through mechanical deformation applied in a die press, hydrostatic press, a forge or an extrusion process.
Another way to deal with the alumina film problem is to mix the aluminum powder with a sintering aid prior to forming the powder into a shape, and then heat the shape to a temperature at which the sintering aid causes a liquid to form that attacks the alumina film. N. Myers et al., in “Rapid Prototyping of Aluminum by Selective Laser Sintering,” Proceedings of 2002 International Conference on Metal Powder Deposition for Rapid Manufacturing, MPIF, Apr. 8-10, 2002, San Antonio, Tex., pp. 233-241, describes mixing magnesium powder into aluminum powder, solid free-form fabricating the mixture into a shape, and then heating the mixture to achieve a highly dense, metallurgically bonded article. Myers reports that the addition of powdered tin into the mixture of aluminum powder and magnesium powder facilitates the densification of the powder mixture. Other sintering aids are mentioned in U.S. Pat. No. 5,640,775, issued to Hayashi et al. on Oct. 24, 1995.
Some persons in the art also have made use of magnesium's high affinity for oxygen as a way of dealing with the alumina film on aluminum powder. Magnesium is one of the few elements that reacts more strongly with oxygen than does aluminum. Also, magnesium alloys favorably with aluminum.
The aforementioned patent to Hayashi et al. teaches using rapidly solidified aluminum powder containing 0.4 to 4.0% by weight of magnesium in combination with special processing to achieve a high density, useful article. It teaches that the aluminum powder is compacted together by the application of mechanical force. The compacted aluminum is sintered by generating nitrogen compounds on the aluminum powder surface through heating the compacted aluminum in an atmosphere having a partial pressure of nitrogen of at least 81 kPa, a partial pressure of a reducing gas that acts as nitrogen-combining acceleration gas component of at least 1 kPa, and partial pressure of water vapor of no more than 1 kPa. Hayashi et al. theorize that simultaneously effecting the reforming of the powder surface by addition of magnesium and the combining reaction with atmospheric nitrogen makes it possible to accelerate the sintering phenomena of the aluminum powder. Hayashi et al. warn that it is necessary to suppress the water vapor partial pressure to 1 kPa or less because water vapor impairs the effect of the magnesium and acts to decompose the nitrogen compound that forms on the powder surface.
U.S. Pat. No. 5,525,292, issued to Nakao et al. on Jun. 11, 1996, teaches the use of sublimed magnesium, i.e., magnesium that has evaporated from the solid state into a gas. Nakao et al. first presses aluminum powder together to form a compact. The compact is heated in a rare gas, such as argon, at a pressure of about one atmosphere (101 kPa). When the temperature reaches 500° C., the pressure is reduced to about 1 kPa or less for a few minutes causing the magnesium present to sublime. The magnesium may be present either as a solid piece of magnesium or magnesium powder mixed in with the aluminum powder in the compact. The magnesium can also be present as part of the aluminum alloy of the aluminum powder, so long as the alloy's magnesium content is at least 0.3% by weight. Nitrogen gas is then introduced to bring the pressure back to about one atmosphere (101 kPa) as the heating is continued to the sintering temperature, e.g., about 540° C., and held there while the compact densifies by sintering. Nakao et al. believe that the sublimated magnesium reacts to generate magnesium nitride (Mg3N2), which reacts with the aluminum oxide on the aluminum powder surfaces to expose metallic aluminum and thereby permits the compact to sinter.
Japanese Patent Laid-Open Publication Hei 06-033164 also teaches the use of aluminum powder that contains magnesium and heating in a nitrogen atmosphere. In this case, the magnesium is used to promote the formation of aluminum nitride (AlN) on the surface of the aluminum powder particles in the temperature range of 500° C. to 600° C. The powder is then compacted together by plastic deformation which breaks up and disperses the aluminum nitride coating within the aluminum shape that is produced.
Another way to deal with the alumina film problem is to forego metallurgically bonding the particles directly to one another and to use a separate binder, for example, an epoxy or a low melting metal or alloy, to bond the aluminum powder together into an aggregate.
The existing methods for circumventing the problem presented by the alumina film on aluminum powder are useful within their bounds. However, it would be advantageous to have a simpler process that does not require either deforming the aluminum powder, using a sintering aid, subliming magnesium at a low pressure, or making the final article as a cemented-together aggregate.